Combustors are commonly used in industrial and commercial operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines and other turbo-machines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, multiple combustors around the middle, and a turbine at the rear. Ambient air enters the compressor as a working fluid, and the compressor progressively imparts kinetic energy to the working fluid to produce a compressed working fluid at a highly energized state.
The compressed working fluid exits the compressor and flows through one or more fuel nozzles and/or tubes in the combustors where the compressed working fluid mixes with fuel before igniting to generate combustion gases having a high temperature and pressure. In particular configurations, each combustor includes multiple bundled tube or micro-mixer type fuel nozzles. The multiple bundled tube or micro-mixer type fuel nozzles are configured to allow premixing of fuel and working fluid (i.e. air) upstream from a combustion chamber prior to combustion. The combustion gases flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
At particular operating conditions, some combustors may produce combustion instabilities that result from an interaction or coupling of the combustion process or flame dynamics with one or more acoustic resonant frequencies of the combustor. For example, one mechanism of combustion instabilities may occur when the acoustic pressure pulsations cause a mass flow fluctuation at a fuel port which then results in a fuel-air ratio fluctuation in the flame. When the resulting fuel/air ratio fluctuation and the acoustic pressure pulsations have a certain phase behavior (e.g., in-phase or approximately in-phase), a self-excited feedback loop results. This mechanism, and the resulting magnitude of the combustion dynamics, depends on the delay time between the injection of the fuel and the time when it reaches the flame zone, known in the art as “convective time” (Tau). Generally, there is an inverse relationship between convective time and frequency: that is, as the convective time increases, the frequency of the combustion instabilities decreases; and when the convective time decreases, the frequency of the combustion instabilities increases. In the case of a bundled tube fuel nozzle, convective time is generally measured as the time it takes for the fuel and air to reach an outlet of the tube as determined from a point within each tube where the fuel is injected.
It has been observed that, in some instances, combustion dynamics may reduce the useful life of one or more combustor and/or downstream components. For example, the combustion dynamics may produce pressure pulses inside the fuel nozzles and/or combustion chambers that may adversely affect the high cycle fatigue life of these components, the stability of the combustion flame, the design margins for flame holding, and/or undesirable emissions. Alternately, or in addition, combustion dynamics at specific frequencies and with sufficient amplitudes, that are in-phase and coherent, may produce undesirable sympathetic vibrations in the turbine and/or other downstream components.
Current systems and/or methodologies for mitigating combustion dynamics include damping systems which are designed to mitigate one particular frequency and/or a limited frequency range. Other systems related to bundled tube fuel nozzles include varying the length of the individual tubes downstream from a fuel plenum portion of the bundled tube fuel nozzle, thus effecting the convection time to mitigate or prevent certain frequencies from occurring within the combustor. However, current systems are generally complex and may be costly to manufacture and maintain. Accordingly, an improved bundled tube fuel nozzle that is configured to mitigate combustion dynamics within a combustor would be useful.